Ambient, Bias-Free, Green Nanoparticle Synthesis via Microdroplet-Confined Galvanic Chemistry.

Nanoparticles have advanced catalysis, sensing, and biomedical engineering, yet their synthesis often requires complex processes, harsh conditions, or applied electrical bias. Here, we introduce a green, spontaneous, and bias-free method for the direct deposition of metal nanoparticles on conductive substrates under ambient conditions. In this process, microdroplets containing aqueous metal salts are sprayed onto a conductive substrate templated with zero-valent Zn microparticles. Within each microdroplet, a confined galvanic exchanged reaction generates Cu, Ag, Pt, and Au nanoparticles with mean diameters below 61 nm. Mechanistic studies indicate that nanoparticle growth arises from the coupling of localized galvanic exchange and lateral charge displacement across the underlying conductor. Unlike recent microdroplet-based syntheses yielding free-standing nanoparticles, our method forms substrate-bound nanoparticles and alloy nanoparticles, without external energy input, non-aqueous solvents, or external bias, and within minutes. By merging the foundational principles of galvanic chemistry with confined reactivity in microdroplets, these results open a path toward more energy-efficient and environmentally sustainable synthesis of functional nanomaterials.